Helium-3 is a rare isotope of the noble gas with applications in quantum …

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The annual meeting of the American Association for the Advancement of Science just took place in Washington, DC, located in a convention center just opposite my favourite bar. I'm going to start off my coverage with news of a resource shortage you may not have heard of, but one with some wide-ranging implications for national security, supercool physics, and pulmonary research. What do these three quite different fields have in common? The answer is helium-3, or 3He, and the problem is there's just not much of it left.

Regular 4He, the kind that makes your voice sound funny and your kid's balloons float, is the first of the noble gases. It has two protons, two neutrons, and two electrons. 3He, however, has two protons but only one neutron, which gives it several interesting properties. First, it can be used as a neutron detector, since 3He has a large collision cross-section for neutrons. When a neutron meets an 3He atom, they react to form tritium (3H, an isotope of hydrogen with one proton, one electron, and two neutrons), and a hydrogen atom (1H, one proton and one electron), giving off energy in the process. There are several applications for sensing neutrons, the main being nuclear threat detection.

Secondly, 3He can be supercooled to temperatures very close to absolute zero, at which point it becomes a superfluid capable of being measured in microKelvins (µKs). With zero viscosity, superfluid 3He will actually climb up and over the walls of its container, which is a neat party trick, but also gives physicists insights into fundamental quantum behaviors. Both the 1996 and 2003 Nobel Prizes in Physics were awarded for research on superfluid 3He. Many refrigeration systems also use the gas to bring other substances down to near absolute zero.

Finally, 3He is a nontoxic gas with a high diffusion coefficient, so it's proven to be quite useful in lung imaging studies. Using MRI machines, it's possible to use 3He to quantify ventilation in different regions of the lung, allowing researchers to detect differences in lung function (say, between normal, asthmatic, and COPD lungs) with far greater spatial and temporal resolution than other methods. It's even possible to measure the microstructures of individual alveoli, the smallest compartments in the lung.

If this were August 2001, there'd be one less Nobel prize to talk about, but more importantly this post wouldn't have needed to be written at all. Although the only source of 3He its appearance as a by-product of nuclear weapons maintenance, there was more than enough of the stuff being stockpiled, and the Department of Energy was hard pressed to sell it for more than $100 per liter. In September of 2001, however, a number of people did something wicked over the skies of America, and all that changed.

The demand for nuclear detectors exploded (if you'll pardon the expression) from 8,000l/year to ten times that in 2008. A once-large stockpile rapidly dwindled and, to make matters worse, the reduction in the US nuclear weapons inventory and half-lives doing their thing have meant that production has been utterly outstripped by demand.

So what can be done about the problem? Luckily, quite a few efforts are underway. Although the national security applications account for 95 percent of US 3He use, there are other ways to achieve the same end. Joe Glaser, from the National Nuclear Security Agency (NNSA), spoke about how this shortage has led to new science. NNSA has a number of different requirements for neutron detectors, from large portal monitors that are being installed in border crossings, seaports, and airports as part of the Second Line of Defense Program, to rugged portable units that can be used in the field.

Truck weigh-and-inspection station with radiation portal monitor.

Oak Ridge National Laboratory

For the static radiation portal monitors, like the one pictured at right, a number of solutions present themselves. Instead of 3He-filled tubes, BF3 can be used, if the boron has been enriched to around 90 percent 10B. These tubes are less sensitive than 3He; you need three tubes of BF3 to do the same work as a single 3He tube, and BF3 is a rather nasty gas, but it's readily available. Lining the detector tubes with a thin film of 10B allows you to avoid working with BF3, again relatively cheaply, although again these detectors are less sensitive than 3He.

Moving away from 10B, glass fibers doped with 6Li have a number of cool features. When neutrons meet the 6Li atoms, the resulting energy gets transferred into the fibers, which we can detect as light (just like the optical fibers that pipe sound between your hi-fi components). They detect both neutrons and gamma rays, and can be made in a range of shapes and sizes, including backpack systems.

Other interesting technologies that are further away from the market include new organic materials that can detect high-energy neutrons. Additionally, NNSA has caught the recycling bug, and believe that it can meet up to 20 percent of its needs by recycling old 3He tubes.

Sadly, unlike neutron detection, the nonsecurity applications of 3He don't have any replacements as Jason Woods of Washington University mentioned as he discussed the impact of the shortage on both low-temperature physics and MRI work. When the supplies of 3He ran out, it essentially put a stop to new science in some areas of low-temperature physics. The US government is rationing out its supplies of 3He, and groups with existing refrigerators are ahead of the threat detection people in the line, but work in this area will be slow going for a while.

For the medical imaging uses, alternatives like 129Xe have been tried with unsatisfactory results. But, happily, it turns out that exhaled 3He is quite easy to recover and recycle. Exhaled 3He is temporarily stored in a He-proof bag—the atoms are so small you can't use just any storage—and then purified cryogenically at 77K.

As for solving the shortage, a number of options exist, but almost none are economically viable. The nearest, most abundant source of 3He is our very own moon, but we'll have more than a little wait before regular shipments start flowing.